Mark libtinyxml2 vendor_available.

libtinyxml2 belongs to vndk-cap. Mark it vendor_available to enable vndk
abi stability checks on it.

Details: https://android-review.googlesource.com/368372

Test: mm -j64

Bug: 38244611

Merged-In: Ida9a8312615f6b8a4b6417d04353bd63a0bbbbcb
Change-Id: Ide7ea89ae22037b75fd70d6238c76b76b5d1a923
(cherry picked from commit 5c495c47)